1. Field of the Invention
The present invention relates to fiber optic pressure sensors, and more specifically, it relates to methods for reducing the size of fiber optic pressure sensors.
2. Description of Related Art
Optical fibers have been used widely for sensing various physical or chemical entities. In many applications, optical fiber is used because it is very thin, being typically 125 xcexcm (0.125 mm). One example is an engine cylinder pressure sensor to be embedded in a spark plug. In such an application, the diameter of the whole sensor should be less than 1 mm. A typical sensor for such an application consists of a tubing with a planar membrane attached at the end and an optical fiber inserted inside. A light from an optical fiber impinges on the membrane and a part of the light is reflected (either by a simple reflection or by an interferometric effect) from the membrane and is coupled back into the fiber and/or other fibers. One major fabrication difficulty in this sensor type is the attachment of the membrane to the end of the small tubing. When the tubing size becomes very small such an operation becomes impossible. Even when attachment can be done, the production cost becomes higher as the size gets smaller. These problems have hindered the wider application of optical fiber sensors.
Among many advantages of using optical fibers for sensors, one distinct advantage is the thinness of its diameter. An optical fiber is typically made of fused quartz, and its typical outer diameter (OD) is 125 xcexcm (=0.125 mm). The OD may be reduced even further to 60 xcexcm or even to 20 xcexcm since the light-guiding core is 50 xcexcm for the commonly-used multimode fiber, and less than 9 xcexcm for the most commonly-used fibers, namely single-mode fibers. Accordingly, there are a variety of applications where a long and thin fiber strand is inserted into a location that is otherwise inaccessible, to measure some physical or chemical entity. One sensor configuration is depicted in a highly schematic fashion in FIG. 1, in which an optical fiber 1 is inserted into a thin tubing 2 that is terminated by a planar membrane 3. FIG. 2 shows the sectional view of the embodiment of FIG. 1. Even though FIG. 1 shows only one strand of optical fiber, it is to be understood throughout this invention disclosure that there can be more than one fiber involved, such as a bundle of fibers. This understanding does not alter the validity and generality of the present invention.
The sensing mechanisms can vary. In some cases, the membrane 3 may be exposed to a certain gas or liquid that produces a fluorescence light emission. The fiber 1 would collect the emission and the wavelength content of the collected light provides information on the nature of the gas or liquid: identification, concentration and/or temperature. In other cases, the membrane may be deflected by an external stimuli, such as acoustic wave, gas pressure, liquid pressure, or physical pressure. The deflection may be detected by first sending light through the fiber 1 and then directing the light returning through the fiber or fibers 1 onto a detector. One common physical mechanism to induce change in the amount of the returned light is Fabry-Perot interferometry. The other is by amplitude modulation. The present invention works with any of these mechanisms, and thus the particularly of the sensing mechanism or sensing entities is not the subject of this invention. With this point understood, the present invention will be described using one simple sensing mechanism, namely the amplitude modulation. FIG. 3 shows a fiber 1 with the core 5 terminated near the membrane 3 (FIG. 3 is actually a close-up view of FIG. 2 near the fiber end). The light impinges on the membrane 3 and is reflected. In this process, only a part of the light 6 is coupled back to the core 5. There could be fibers, other than the input fiber 1, for collecting the reflected light so that the fiber 1 is used only for transmitting the light 6 to the membrane 3. This is for instrumentation convenience. Now, referring to FIG. 4, if the membrane 3 is deflected, the pattern of the light reflection is altered, and the amount of the light 6 being coupled back into the fiber 1 changes. The amount of the change indicates the amount of the deflection. (In an interferometric sensor, the change in the gap modulates the light reflection; a change by one-quarter wavelength causes a full swing between the maximum and the minimum in the reflected light power. In this case, only one fiber 1 is used for sending and collecting the light 6.)
The market size of fiber optic sensors is very large, exceeding a few hundreds of millions of dollars today, and it is still expanding rapidly. One technical stumbling block is the fabrication. When the size of the tubing becomes smaller than about 1 mm, it becomes more difficult to attach the membrane 3 at the end of the tubing 2. And there are many applications in which small size is essential.
It is an object of the invention to make to reduce the size of fiber optic pressure sensors.
It is another object of the invention to reduce the cost of fabricating small-size fiber optic pressure sensors.
The difficulty in reducing the size of fiber optic pressure sensors is mainly related to the requirement that the membrane should be smooth and taut. The smoothness is required for satisfactory light reflection, and the tautness is required for reproducibility, quick response, and also satisfactory reflection. This difficulty can be lessened substantially if the membrane is designed to have a non-planar surface in its natural state (in the absence of a stimulus such as a pressure), such as spherical, conical or wedge shape. Then some of the conventional fabrication methods such as extrusion and forming can be mobilized much more readily for the sensor fabrication.
It may be necessary in some applications (when the membrane material is too thin or too flexible) to transform the area under a non-planar membrane into a cavity using a plugging material, so as to make the enclosed space air-tight. Then the non-planar membrane can keep its shape and remain taut like a balloon surface as the surface becomes resilient due to the air-cushion effect of the air-tight cavity.
As an example of fabrication methods, one can literally blow a balloon, in which a molten or liquid state material is blown by a pressure applied to the other end of the tubing until it forms a semi-sphere. The extra amount of the molten or liquid state material will be blown out of the tubing, leaving behind a thin balloon-like membrane. The material is then solidified, either (i) thermally using a heater, (ii) as the molten material cools down, or (iii) by polymerization by ultra-violet (UV) exposure if it is a UV-curable polymer. Liquid rubber material called RTV, which solidifies by itself over a time period, is another excellent raw material for making the novel membrane. A low melting-temperature glass may be blown into the sphere inside a high-melting temperature tubing such as quartz or tungsten. The spherical nature of the surface does not have to be pronounced. So long as the surface is even slightly convex, it will be much easier to fabricate the membrane following the teaching described here.
Another embodiment with a great potential uses a small ball that is made separately and then inserted inside the tubing, or attached to the tubing. There are conventional fabrication methods for making miniature size spheres or balls, and fiber optic sensors can utilize such spheres. The surface of the ball that is exposed outside the tubing will work as the membrane. In many applications, the membrane should be thin. However, there are other applications in which the stimulus is very powerful. An example is the engine cylinder pressure sensor. The pressure can reach about 1,000 psi. In such applications the ball should be solid.